1. Field of the Invention
The present invention relates to an image processing apparatus, such as a digital camera or a digital video camera, which is adapted to correct an image signal by using data for correction.
2. Description of the Related Art
In recent years, an image processing apparatus, such as a digital camera, using an image pickup device, such as a CCD (charge-coupled device) sensor or a CMOS (complementary metal-oxide semiconductor) sensor, has been used for various purposes. In the process of or after manufacturing an image pickup device, a defect may occur at some pixel thereof. That is, some defective pixel, which outputs a signal having an abnormal signal level, may occur in the image pickup device.
A method of preliminarily storing the address of a defective pixel in an image pickup device and a defect level and an output level at the defective pixel and of correcting image information at the stored address through interpolation with information at surrounding pixels has been proposed as a method of correcting a defective pixel.
This correction method is more specifically described hereinbelow with reference to FIGS. 4A, 9, and 10.
FIG. 9 is a block diagram showing the configuration of an image processing apparatus, such as a digital camera or a digital video camera.
As shown in FIG. 9, the image processing apparatus includes a digital signal processor (DSP) 1 including an A/D converter configured to perform analog-to-digital (A/D) conversion on a signal output from an image pickup unit 2, a signal processing circuit configured to perform signal processing that includes a signal correction, a circuit configured to control storage operations of various kinds of storage units, a transfer unit configured to transfer data, and a circuit configured to control transfer operations. The image pickup unit 2 includes an image pickup device (not shown), such as a CCD sensor or a CMOS sensor, configured to convert light, which comes from a lens that forms an image of an object, into an electrical signal. An external memory 3 serves as a storage unit that stores a shot image. A flash read-only memory (hereunder abbreviated as FROM) 24 stores control programs and various data used in the image processing apparatus. A synchronous dynamic random access memory (SDRAM) 25 temporarily stores data when the DSP 1 performs signal processing because the SDRAM can be accessed faster than the FROM 24.
A central processing unit (CPU) 10 controls the DSP 1 and also controls components adapted to perform other image processing functions, such as photometering and distance measurement. A main power switch 11 is operable to activate the image processing apparatus. A release switch 12 is operable to start the image processing functions of the image processing apparatus. Each of the switches 11 and 12 is turned on by being shortcircuited to ground (GND).
FIG. 10 is a flowchart illustrating a control operation performed by the CPU 10 in the image processing apparatus shown in FIG. 9. First, it is determined in step 801 whether the main power switch 11, which activates the image processing apparatus, is on. If the power switch 11 is off, step 801 is repeated. On the other hand, if the power switch 11 is on, a power supply (not shown) for the image processing apparatus is activated in step 802. Also, the CPU 10 stands by until the power supply is stabilized. Upon completion of standby until the power supply is stabilized, data, which is obtained by the image pickup device and is stored in the FROM 24, is transferred to the fast accessible SDRAM 25 in step 803. Consequently, preparation is made for fast accesses to the data by the DSP 1 during signal correction.
The data, which is obtained by the image pickup device and is stored in the FROM 24, includes defective pixel information on a defective pixel in the image pickup device. One piece of defective pixel information represents the address and the defect level of one defective pixel. The FROM 24 stores pieces of defective pixel information, the number of pieces of which is equal to that of defective pixels.
In step 804, it is determined again whether the main power switch 11 is on. If the power switch 11 is off, this operation of the image processing apparatus is finished. On the other hand, if the power switch 11 is on, it is determined in step 805 whether the release switch 12 is on. If the release switch 12 is off, processing returns to step 804. On the other hand, if the release switch 12 is on, the photometering and the distance measurement are performed in step 806. Also, if necessary, a shooting lens (not shown) of the image processing apparatus is driven.
According to results of the processing performed in step 806, the CPU 10 determines image-taking conditions, such as the moved position of the shooting lens, a diaphragm value, an accumulation time (storage time) of the image pickup device, and a reading gain at the time of reading of a signal from the image pickup device, in step 807. Then, in step 808, a storage operation of the image pickup device is started.
Assuming that both the power switch 11 and the release switch 12 are simultaneously turned on, a time, which is taken since the power supply of the image processing apparatus is turned on in this state in step 801 until the storage operation of the image pickup device is started in step 808, is defined as a starting time of the image processing apparatus.
Subsequently, in step 809, the storage operation of the image pickup device is stopped after the lapse of the accumulation time (storage time) determined in step 807. Then, in step 810, a signal output from the image pickup device is read in a state in which a gain is set at the reading gain determined in step 807. Upon completion of or while reading a signal output from the image pickup device, the DSP 1 performs various kinds of correction, such as correction of a defective pixel, according to the defective pixel information, which is stored in the SDRAM 25, in step 811.
Subsequently, in step 812, corrected image data is stored in the external memory 3 (hereunder also referred to as a recording medium). Then, processing returns to step 804. Thereafter, processing of steps 804 to 812 is repeated. The corrected image data may be displayed by a display unit (not shown) at that time. Steps 804 to 812 are repeated until it is determined in step 804 that the power switch 11 is off.
FIG. 4A diagrammatically illustrates the flow from power-on to the reading of signals (including the correction processing thereof), which is shown in FIG. 10 and described above.
After the power supply is turned on, the CPU 10 stands by until the power supply is stabilized. Then, all the data including the defective pixel information, which is stored in the FROM 24, is transferred to the SDRAM 25. Subsequently, the storage operation of the image pickup device is performed. Then, the reading of signals is performed. The starting time of the image processing apparatus is the sum of a time during which the CPU 10 stands by until the power supply is stabilized and a transfer time of all the data (see, for example, Japanese Patent Application Laid-Open No. 3-227185).
However, in recent years, image pickup devices, each of which has a great number of pixels, have been realized. Thus, there is a tendency that the number of defective pixels in an image pickup device increases. Consequently, the number of defective pixels stored in the FROM 24 is enormous. The transfer time of the defective pixel information is unignorably long. Therefore, much time is needed for performing only step 803 shown in FIG. 10. Thus, a photographer may miss a good opportunity for taking a picture of a target scene. Also, when data stored in the FROM 24 is transferred after one picture is taken, a subsequent picture cannot be taken immediately after a first picture. Consequently, a photographer may miss a good opportunity for taking a second picture.